This invention relates to a noise attenuation device, and in particular to a compact noise attenuation device.
International application No PCT/AU98/00676 the contents of which are incorporated herein by reference. is concerned with the provision of a device that acts to attenuate noise entering a building through a natural ventilation opening. such as a window which allows the occupants of that building to enjoy the benefits of natural ventilation whilst not being subject to undesirable levels of noise.
The international application discloses the use of arrays of quarter wave resonators disposed around a ventilation opening, specifically a partially blocked window. Typically, the resonator arrays are positioned around the ventilation opening. For example, in FIG. 3 of PCT/AU98/00676, they are shown attached to the outside wall of a room to be ventilated around a window in an array in which the resonator which is tuned to the lowest frequency is closest to the wall/opening and the resonators which are tuned to the highest frequency are located furthest from the opening.
The present invention is concerned with improvements in the design and function of the array to provide an improved noise attenuator which may also be used in other applications. For example, the majority of residences in Australia are naturally ventilated rather than sealed and air-conditioned. As a consequence, building facades contain fixed air vents. In older buildings, these fixed air vents are approximately 250 mm by 170 mm each in size, with an typical open area for ventilation of less than 10%. The total area of the fixed vent is one standard brick length by two brick heights. Each room in a typical residence contains at least two vents, located in walls forming the building envelope. The vents are important to maintain human comfort inside the residence by providing adequate ventilation, to ensure satisfactory air flow throughout the residence, to prevent mould growth and to allow gases emitted by furniture to escape. It is desirable to improve the vent such that air flow is maintained or improved but sound transmission reduced.
Another problem, identified by the inventor, is noise produced by air conditioning in offices. Most offices have suspended ceilings. In one popular design, elongate narrow outlets extend along the sides of fluorescent light fittings (large rectangular boxes typically containing two light tubes, sockets and ancillary equipment). Noise from the air conditioning fan and regenerated noise from associated system components is transmitted through air outlet into the office below. In some cases vents are provided adjacent light fittings which are not connected to air conditioning ducts but simply allow a return air path for air to enter the ceiling space. Such vents also act as a noise transmission path and allow voices in particular to travel from one office to another.
It is the aim of the present invention to address the problems discussed above and provide improved noise attenuation devices.
Thus in a first broad aspect of the present invention, there is provided a noise attenuation device for attenuation of noise passing along a vent having a width the array comprising a plurality of rows of tubes having a mouth width w and a length L, the rows being arranged in parallel in side by side relation, and each array including tubes having different mouth widths and lengths so that at least some of the rows of tubes in each array are tuned to a different resonant frequency to others of the rows of tubes in that array;
the open mouths of the tubes being contiguous a gap or ventilation opening having a width H and wherein the tubes satisfy the relation:
w greater than H.
To ensure the performance of the device is satisfactory, it is preferred that for the majority of the tubes in the arrays, each ratio of individual tube equivalent diameter (D) to its length (L) (the xe2x80x9cscalexe2x80x9d) satisfies the following relation:
D/L less than 0.25.
For a square tube D=2w/{square root over (xcfx80)}, where w is the side width of the square tube.
The device may be optimised for particular applications, for example for natural ventilation in residences as discussed in the introduction.
Thus, according to a second aspect of the present invention there is provided:
a noise attenuation device for insertion in a ventilation aperture in a wall of a building or the like, including a first noise attenuation element comprising an array of quarter wave resonators and a second noise attenuation element comprising an array of quarter wave resonators;
a noise pathway disposed between the first and second noise attenuator elements, each array comprising a plurality of rows of tubes having a mouth width w and a length L, the rows ben arranged in parallel in side by side relation, and each array including tubes having different mouth widths and lengths so that at least some of the rows of tubes in each array are tuned to a different resonant frequency to others of the rows of tubes in that array;
the two arrays being separated by an aperture or ventilation opening having a width H extending from one array to the opposite array;
wherein the aperture is kinked or curved so there is no direct line of vision through the aperture perpendicular to the face of the device.
Fixed air vents in buildings provide an airborne transmission path for noise. By replacing the conventional air vent with the attenuator, noise entering the building through the vent must xe2x80x9cinteractxe2x80x9d with the device.
The present invention also allows an array of vents to be built into a wall where significant air movement is required and thermal comfort is of high priority, in this case several noise attenuators can be used in side by side relation.
The provision of the kink in the attenuator provides an indirect sound path from outside to inside of the building via the fixed air vent ie the air path between the inlet and outlet of the attenuator is not straight. This reduces the sound passing through the device by providing a multiple barrier diffraction effect. Also the angled opening means that the open mouths of the tubes are angled. This provides two significant advantages. First, the angled mouth has a larger cross-sectional area than a conventional tube opening, increasing the useful area used for the desired scattering mechanism. Secondly, in relation to the first effect, the grazing incidence of sound passing the open mouths of the tubes is lessened by the angled opening. The scattering mechanism is most efficient at normal incidence for sound and least efficient for grazing incidence. The angled mouths provide improved performance over grazing incidence. The lack of a direct line of sight through the barrier also has positive implications with regards to building security.
To ensure the performance of the device is satisfactory, it is preferred that for the majority of the tubes in the arrays, each ratio of individual tube equivalent diameter (D) to its length(L) (the xe2x80x9cscalexe2x80x9d) satisfies the following relation:
D/L less than 0.25.
For a square tube D=2w/{square root over (xcfx80)}, where w is the side width of the square tube.
This relationship has been based on experiments by the inventor involving the measurements of the frequency response of individual tubes of varying scale. It was found that for tubes not satisfying this relationship, the quality factor (Q) of each of the tubes was not high enough to be most effective as a scatterer.
It is preferred that when the device is installed in a building, the device is arranged such that the tubes having smaller mouth widths are located on the side of the device facing the outside of the building. Tubes with larger mouth widths should be located towards the side closest to the inside of the building. Hence the tubes are to be arranged in order of ascending length (or mouth width) from the side closest to the outside of the building.
It is preferred that tubes of similar mouth widths are located opposing each other on each side of the kinked ventilation aperture.
However, it has been found that tubes with equivalent diameters (D) greater than the width of the ventilation opening (H) where they are located do not require tubes on opposing sides of the ventilation opening. Thus, when the relation D greater than H applies, then tubes of that diameter only need to be located on one side of the ventilation opening ie, in one of the attenuation elements.
The width of the ventilation aperture will also determine the smallest equivalent tube diameter in the array.
The performance of tubes tuned to high frequencies is most sensitive to the ventilation opening dimensions as shorter wavelengths are involved. The distance from the opposing open ends of individual tubes tuned to higher frequencies where the scattering mechanism is useful is much shorter than for tubes tuned to lower frequencies, which may be demonstrated from the derivation of total energy of an individual tube cavity.
Since the tubes tuned to the highest frequency have the shortest wavelength, the performance of these tubes are determined by the ventilation opening width. It is preferred that the length of the smallest tube, should therefore satisfy the following relation:
L greater than H/2
Although tubes not satisfying the above relation would be expected to produce some desired scattering effects, they would not be expected to perform as effectively.
To improve the compactness of the device, the tube with largest mouth width may include an initial straight portion and a second portion which extends at a right angle. The following criteria must be satisfied for the kinked tube to perform effectively:
D greater than H
The length of the tube which is perpendicular to the main or initial length must be less than the initial straight length of tube.
In a third aspect of the present invention, there is provided a noise attenuation device for attenuation of noise passing along a vent having a width the array comprising a plurality of rows of tubes having a mouth width w and a length L, the rows being arranged in parallel in side by side relation, and each array including tubes having different mouth widths and lengths so that at least some of the rows of tubes in each array are tuned to a different resonant frequency to others of the rows of tubes in that array;
a plate disposed opposite the array defining an aperture or ventilation gap having a width H therebetween
wherein the tubes and ventilation gap satisfy the relation:
w greater than H.
The above third aspect of the present invention provides a noise attenuator which is particularly suited to attenuating fan noise in air conditioning ducts and outlets. The tubes are typically arranged in order of increasing frequency from the upper end of the duct closest to the noise source (the air-conditioning fan).
There are also some differences between this second embodiment and the first embodiment. The tubes should be tuned to the fan noise which typically produces lower dominant frequencies to attenuate. This means larger tube widths are required. In fact, in some applications, tubes widths much larger than those illustrated in FIGS. 9 to 11 may be utilised.